The present invention relates to an input signal detection device for detecting whether or not an optical signal has been inputted to an optical device such as an optical amplifier in an optical communication system and an optical device control apparatus using the result of the detection.
FIG. 28 shows a configuration example of a related art optical communication system using the wavelength division multiplexing (WDM) technology. This optical communication system includes an optical transmission device 11, transmission line fibers 12, 14, and 16, optical amplification relays 13 and 15, and an optical reception device 17.
Among these, the optical transmission device 11 includes optical transmitters 21-1 to 21-n, an optical multiplexer 22, and an optical amplifier 23. The optical reception device 17 includes an optical amplifier 24, an optical demultiplexer 25, optical amplifiers 26-1 to 26-n, variable wavelength dispersion compensators 27-1 to 27-n, and optical receivers 28-1 to 28-n. The optical amplification relays 13 and 15 and the optical amplifiers 23 and 24 each amplify a WDM signal as a single unit, while the optical amplifiers 26-1 to 26-n each amplify an optical signal of one wavelength.
One of optical amplifiers that are currently most widely used is an erbium-doped fiber amplifier (EDFA) that uses an induced emission of a rare earth element, erbium, which is added to the core of an optical fiber. The optical amplification relays 13 and 15 amplify optical signals that have been transmitted through the transmission line fibers 12 and 14, respectively, and have reduced their power.
At this time, simultaneously with the amplification of each optical signal, an amplified spontaneous emission (ASE) occurs that has a random amplitude, phase, polarized wave, and the like due to the induced emission. Thus, the optical signal to noise ratio (OSNR) is deteriorated. This ASE is amplified and accumulated each time it passes through an optical amplification relay, and is finally inputted to the optical reception device 17 together with an optical signal.
In an example shown in FIG. 28, light including a WDM signal 31 and an ASE 32 is outputted from the optical transmission device 11. Then, light including a WDM signal 33 and an ASE 34 is inputted to the optical demultiplexer 25 of the optical reception device 17 and light including an optical signal 35 of one wavelength and an ASE 36 is inputted to the optical amplifier 26-2.
A tolerance to wavelength dispersion is significantly reduced in a high-speed optical transmission system having a transmission speed per wavelength of 40 Gbit/s; therefore, a highly accurate wavelength dispersion compensation is needed. For this reason, the variable dispersion compensators 27-1 to 27-n are provided in the optical reception device 17. This allows a highly accurate wavelength dispersion compensation for each channel, as well as allows constant optimization of the amount of dispersion compensation while following temporal variations in wavelength dispersion value with time during operation of the system. Also, if signal quality significantly deteriorates due to polarization mode dispersion (PMD), a PMD compensator may be disposed between the optical demultiplexer 25 and the optical receivers 28-1 to 28-n in order to compensate for such deterioration.
However, application of the variable wavelength dispersion compensators 27-1 to 27-n or the PMD compensator may increase optical loss, thereby causing lack of light power over the input dynamic ranges of the optical receivers 28-1 to 28-n that are disposed after these components. In this case, input power to the optical receivers 28-1 to 28-n is secured by amplifying the optical signals using the optical amplifiers 26-1 to 26-n.
FIG. 29 shows a system for controlling such an optical amplifier for loss compensation. An optical amplifier 42 amplifier is provided before an optical receiver 43 so as to amplify input light. An optical coupler 41 and a photodiode (PD) 44 are provided on the input side of the optical amplifier 42 so as to monitor input light. According to a monitor signal from the PD 44, a processor 45 determines whether or not an optical signal has been inputted. The controller 46 controls operations of the optical amplifier 42 according to the result of the determination.
As shown in FIG. 30, the processor 45 sets a shutdown threshold Pth of light power near the lower limit value of the signal input range. If monitored light power is higher than the Pth, the processor 45 determines that a signal has been inputted. If monitored light power is lower than the Pth, it determines that no signal has been inputted. If a signal has been inputted, the processor 46 causes the optical amplifier 42 to operate; if no signal has been inputted, it causes the optical amplifier 42 to stop operating (that is, it shuts down the optical amplifier 42).
Therefore, if an optical signal is turned off at a time t1 and input light power 51 of the optical amplifier 42 falls below the Pth, the optical amplifier 42 is shut down and output light power 52 of the optical amplifier 42 comes close to zero.
Japanese Laid-open Patent Publication No. 2004-112427 relates to a method for monitoring the OSNR in an optical transmission system.
The above-mentioned related art optical amplifier control method has the following problem.
As shown in FIG. 31, if only one channel of a WDM signal is turned off due to breakage, removal, or the like of a optical fiber of the optical transmitter 21-2 during operation of the WDM communication system having n channels, only an ASE that has occurred and accumulated in the optical amplification relays disposed between the optical transmission unit and the optical reception unit is inputted to the optical amplifier 26-2 corresponding to that channel.
If this ASE power is larger than the lower limit value of the signal input dynamic range of the optical amplifier 26-2, input light power 61 does not fall below the shutdown threshold Pth even if the signal is turned off at the time t1, as shown in FIG. 32. As a result, a distinction cannot be made between the signal and the ASE, whereby the optical amplifier 26-2 will not be shut down.
Then, if the signal is turned on at a time t2 with the optical amplifier 26-2 operational and the optical signal is inputted to the optical amplifier 26-2, an optical surge 63 occurs as shown in output light power 62. Thus the optical receiver 28-2 disposed after the optical amplifier 26-2 will be broken.
Also, if the optical amplifier is mistakenly started when only an ASE has been inputted at an initial start of the WDM communication system, an optical surge occurs at an instant when an optical signal is actually inputted afterward. Thus, the optical receiver will be broken as well.
To prevent such an erroneous determination, a method is considered in which an input signal detection device as shown in FIG. 33 is used. An input signal detection device includes an optical coupler 72, a high-speed PD 73, a band path filter (BPF) 74, and an intensity monitor 75. A monitor signal outputted from the high-speed PD 73 is transferred to the intensity monitor 75 via the BPF 74, and the intensity monitor 75 monitors the intensity of components of the signal and outputs the monitor signal to the controller 76. According to the monitor signal from the intensity monitor 75, the controller 76 determines whether or not an optical signal has been inputted, and controls the operation of the optical amplifier 42.
Monitoring the intensity of the signal components at the input terminal of the optical amplifier 42 in this way allows determination whether a signal has been inputted or only an ASE has been inputted. However, disposing the input signal detection device 71 by the number of wavelengths requires use of many high-frequency parts. This will make the system very costly.